Fueled by exponentially growing demand for broadband services, the transport capacity of next-generation optical access networks will migrate to 40 Gb/s per channel in the near future. However, unlike long haul networks whose the distance-bandwidth product is large enough to leverage high implementation cost, access networks (<100 km) must maintain low hardware and operation costs to remain attractive and practical.
It is well known that in 40 Gb/s optical links, fiber dispersion can severely limit transmission distance. Orthogonal Frequency Division Multiplexing (OFDM) has recently emerged as a very promising modulation format for high-speed optical transmission due to both high resistance to fiber dispersion (both CD and PMD) and high spectral efficiency. By thus eliminating the need for dispersion compensation and reducing the transmission bandwidth, OFDM can significantly increase flexibility of access passive optical networks (PON) while reducing implementation cost. However, since an OFDM signal can only be generated by high speed digital-to-analog converters (DAC), current DAC technology with maximum sample rate of 10 Gsample/s at 8-bit resolution (256 levels) limits OFDM bandwidth to 5 GHz. In order to generate a 40-Gb/s OFDM signal in a 5 GHz bandwidth, 256-QAM modulation would have to be used, yet currently, it cannot be realized at 8-bit DAC resolution. Polarization-Multiplexing (POLMUX), wherein a high-speed OFDM signal is carried in two orthogonal polarizations, has been widely proposed in long-haul OFDM transmission as a spectrally-efficient alternative to generating very high-speed signals. The trade-off in such multiple-input multiple-output (MIMO) POLMUX systems is the need for coherent detection which entails narrow line width lasers and complex frequency-offset and phase noise compensation algorithms that are too costly for access networks.
Accordingly, there is a need for a method that circumvents the above noted difficulties by exploiting polarization multiplexing POLMUX with direct-detection to realize 40 Gb/s transmission over 20 km standard single mode fiber SSMF.